Some conventional amplifiers or buffers include transistors in common-drain configurations. These buffers, commonly referred to as source followers, are advantageous over other amplifier or buffer circuits because of their ability to operate at high speed. Such buffers also enable circuits that provide an output having a high current drive from an input having a relatively low current drive capability. Source follower circuits are also commonly used as level-shifting circuits, for shifting DC bias points in analog or mixed signal circuitry. As illustrated in FIG. 1, a conventional source follower 100 can include a metal-oxide-semiconductor (MOS) input transistor 110 and a current supply 120 that drives a load 130 at the output of the source follower 100. In some implementations, transistor 110 can be a P-channel MOS (PMOS) transistor having a source terminal coupled to current supply 120 and a drain terminal coupled to a ground potential. An input signal VI is applied to a gate terminal of the input transistor 110, and an output signal VO (driving a load 130) is generated from the source terminal of input transistor 110.
There are numerous non-idealities of a typical source follower circuit. For example, the gain of a typical source follower circuit may be characterized by the following Equation (1):
                                          V            O                                V            I                          =                  1                      1            +                                          g                mb                                            g                m                                      +                          1                                                g                  m                                ⁢                                  r                  ds                                                      +                          1                                                g                  m                                ⁢                                  r                  o                                                      +                          1                                                g                  m                                ⁢                                  R                  L                                                                                        (        1        )            where gm, gmb, and rds are the transconductance, the back-gate transconductance, and the source-drain output resistance of the input transistor, respectively, ro is the current source output resistance, and RL is the load resistance. The loss due to the back-gate transconductance can be eliminated by coupling or tying the back gate, or body, of the transistor to the source (thus gmb is zero). However, even assuming ideal conditions (for example, where the current source 120 is ideal and the load 130 is purely capacitive, in which case ro and RL are infinite), the gain is still limited by the source-drain output resistance of the input transistor. Moreover, when the current source output resistance is not infinite, the current through the source-drain terminals of the transistor will be modulated due to voltage swings of the input signal (and thus also of the output signal), causing further gain errors and non-linearities.
A “super” source follower 200 is illustrated in FIG. 2. Source follower 200 reduces the voltage dependent gain error by regulating the current through the input transistor to be nearly constant using a feedback loop. The feedback loop in the source follower 200 includes an N-channel MOS (NMOS) transistor 240 having a drain terminal coupled to the source terminal of the input transistor 210, and a second current source 250 coupled to the drain of the input transistor 210 and the gate of the NMOS transistor 240. The second current source 250 is generally a static current source (i.e., the second current source 250 generally sinks a fixed current). The feedback loop reduces the current source output impedance, thereby improving linearity and also reducing the gain error contribution due to the resistive component of load 230. However, super source follower circuit 200 does not lessen the gain error contribution due to the finite source-drain output resistance of the input transistor 210.
Modern electronics requiring high-bandwidth, high-linearity, and/or low-noise designs challenge the conventional assumption that source followers and super source followers have little effect on the AC performance (for example, gain and frequency response) that is predominately caused by the finite source-drain output resistance of the input transistor.
This “Discussion of the Background” section is provided for background information only. The statements in this “Discussion of the Background” are not an admission that the subject matter disclosed in this “Discussion of the Background” section constitutes prior art to the present disclosure, and no part of this “Discussion of the Background” section may be used as an admission that any part of this application, including this “Discussion of the Background” section, constitutes prior art to the present disclosure.